The invention relates to laser power supplies and, more specifically to high-current, pulse-mode drivers for laser diodes.
Pulsed laser diode drivers are used to generate pulses of current, typically into a series array of lasing diodes. The light output is used for various purposes, such as pumping lasers, or timed illumination. The driver design typically comprises a storage capacitor, the laser diode array, and a pulsed current source, connected in series. When the current source is turned on, energy is drawn from the capacitor through the diode array. The voltage on the capacitor falls, so the current source must have sufficient compliance to continue to operate as voltage falls. For good efficiency, a low voltage loss across the current source is desired, but this requires a large and bulky capacitor to minimize voltage sag.
U.S. Pat. No. 5,287,372 (issued Feb. 15, 1994 to Joseph A. Ortiz, patent hereinafter referred to as xe2x80x9cORTIZxe2x80x9d) discloses a quasi-resonant diode drive current source providing a high power pulsed current that drives light emitting diodes and the like. ORTIZ discloses a zero-current-switched full wave quasi-resonant converter that provides a high amplitude pulsed output current for driving light emitting pump diodes used in a solid state diode pumped laser.
In ORTIZ, a resonant buck converter power supply drives the diode array directly. While this is an effective way to drive the diode array, it has the disadvantage in pulsed applications that the power supply must be designed to handle the peak power required by the diode array, despite the fact that the average power required can be substantially lower. As a result, bulky and expensive power supply components must be employed (e.g., capacitors, transformers, coils, switching elements, etc.).
In pulsed laser diode drivers, the rise time of the current is limited by the inductance of the diode. array, cabling, and laser diode driver components. Typically, slow rise times are improved by increasing the driving voltage to help overcome the inductance of the diode array, cabling, and driver components. Unfortunately, in pulsed current sources, additional voltage results in greater power dissipation in the current source after the initial voltage surge across the laser diode is over and the current settles to the steady-state value after the pulse has risen. Again, this requires larger, more bulky and more expensive drive components that can dissipate greater power and withstand the higher voltages.
The buck regulator method described in ORTIZ has poor rise time performance because the switching pulses of the converter must be integrated and regulated with a feed back loop to generate a smooth and controlled output current. This, in turn, is limited by the switching frequency of the buck regulator, which will usually be far too slow to achieve the extremely fast rise times that are often required for pulsed laser applications.
Evidently, there is a need for a pulsed laser diode driver with fast rise time, good efficiency and small size. Such a driver would be particularly useful for applications requiring narrow pulses and high peak power.
It is therefore a general object of the invention to provide an improved laser diode driver.
It is another object of the invention to provide a pulsed laser diode driver with fast rise time, good efficiency and small size.
It is another object of the invention to provide a laser diode driver which is useful for applications requiring narrow pulses and high peak power.
According to the invention, a diode array driver is connected between a power supply and a diode array. An input of the driver receives power from the power source. An output of the driver delivers power to the diode array. The driver comprises an energy storage inductor between the power supply and an end of the diode array, for storing energy. A switching element, is connected between the other end of the diode array and ground. A shunt switch is connected across the diode array. When the shunt switch is opened, energy stored in the inductor is suddenly delivered to the diode array. A diode may be connected between the other end of the diode array and the input of the driver. A current monitor may be connected in series with the diode array.
The power supply may comprise a voltage source or a current source.
The shunt switch is suitably a saturating-type switch, such as FET, BJT, IGBT or SCR.
The switching element connected between the diode array and ground may be a saturating-type switch (e.g., FET, BJT, IGBT, SCR), or a switched current source.
Means may be provided for returning residual energy stored in the inductor back to the power supply.
A plurality of such drivers may be connected to a single power source for operating a corresponding plurality of diode arrays.
An overall system comprises the diode array driver(s) and at least a portion of the power supplyxe2x80x94namely, an energy storage capacitor. A value for the energy storage capacitor in the power supply may be selected to produce a maximally flat-top pulse shape. A source voltage provided by the power supply may be greater than the voltage required by the diode array (load), it may be substantially equal to the voltage required by the load, or it may be less than the voltage required by the load.
The present invention provides a combination of good efficiency, fast rise time to high currents, and is tolerant of lead and laser diode inductances. In addition, the energy storage capacitor is of much smaller size and the circuit is able to be designed for use with a wide range of input voltages.
In use, a first switch is turned on to build up current in the inductor just prior to the laser pulse. During this period, the load (diode array) is shorted out by a second (shunt) switch in series with the first switch. When the desired peak current is reached, the second switch is turned off (opened). Because current flow in an inductor can not change abruptly, the current continues to flow into the load, generating a high voltage, as required, to overcome the reactance of the load and leads. To turn off the pulse, the second switch is turned on, shorting out the load and discharging the lead/load inductance as the falling edge of the pulse. Simultaneously, the first switch is turned off and the energy stored in the series inductor is recycled back into the storage capacitor through a diode.
The benefits/advantages of the invention include:
Current is forced into the load and a voltage is generated automatically to achieve a fast rise time to high currents.
The laser diode array is normally expensive and susceptible to electrical damage.
It is protected by the shunt switch.
Because energy is stored in the series inductor, diode arrays of higher voltage than the power source may be driven, simplifying the power supply requirement.
The switches are saturated and therefore have a much lower loss than a linear current source, resulting in high overall efficiency.
Extra energy stored in the series inductor is recycled back to power source capacitor.
Energy stored in the laser diode/lead inductor is dissipated in the load and used as part of the pulse energy resulting in a very low ringing or negative voltage swing across the diode array, which could otherwise be harmful.
Multiple arrays can be driven from the same power source and capacitor. A separate series inductor for each array allows the current to be controlled independently by the switches associated with that array.
The storage capacitor and its initial voltage may be chosen to create a partial discharge during the pulse. This prevents the current from continuing to build during the period when switch one is on. The value of the capacitor is adjusted for the flattest pulse-top if the source voltage is higher than the diode array voltage. If the source voltage is less than the diode array voltage, then the series inductor must provide the current flow during the pulse and be sized accordingly.
Other objects, features and advantages of the invention will become apparent in light of the following description thereof.